Driven by bandwidth hungry applications, optical broadband access networks have advanced very rapidly in recent years, becoming the core of new triple-play telecommunication services, which deliver data, video and voice on the same optical fiber right to the user. Deep penetration of the optical fiber into the access networks is accompanied with massive deployment of the optical gear that drives data traffic along the fiber links. The result is that wavelength division multiplexed optical networks which receive downstream and send upstream data signals using multiple optical signals on a single optical fiber are now being deployed at every optical line terminal or/and network user interface rather than their historical deployments within the long-haul network and infrastructure backbone networks.
Such deployments range from the provisioning of only two or three wavelengths, in the case of Fiber-to-the-Home, albeit with volumes of millions of units as one is required for every subscribers home, through to those provisioning typically 4, 8, or 12 wavelengths in the local loop and router feed networks, to those providing 16, 20, 32, 40 and more wavelengths in the metropolitan area networks and long-haul networks.
Further, the carrier roadmaps of initially provisioning broadband optical access, e.g. Broadband Passive Optical Network (BPON), with subscriber downstream/upstream at 10 Mb/s to 100 Mb/s, evolving through Ethernet based access, e.g. Ethernet Passive Optical Network (EPON), and on to Gigabit Passive Optical Network (GPON) wherein 2.5 Gb/s is provided downstream per subscriber and upstream supports 1.2 Gb/s transmission. Competing roadmaps from national carriers outside the United States such as Japan and Korea are developing Wavelength Division Multiplexed Passive Optical Network (WDM-PON), wherein each subscriber has a discrete wavelength provisioned to them supporting potentially bidirectional 2.4 Gb/s transmission. Such roadmaps very quickly limit even the capacity of today's largest 80 channel 10 Gb/s communications backbone networks. At maximum streaming with GPON such a link potentially only supports 320 subscribers!
Therefore, cost efficiency and volume scalability in manufacturing of the components within such wavelength division multiplexed networks (WDM) are increasingly becoming the major requirements for their mass production. Further, where photonic integrated circuits (PICs), also referred to a integrated optical components or circuits, is considered for the provisioning of the functional elements there is considerable benefit from providing the optical elements within a design environment that supports the integration of potentially optical and electrical circuits within a single integrated circuit.
Hence PICs, in which different functionalities are monolithically integrated onto one photonic chip, are an attractive technology and component solution in that they enable the production of complex optical circuits using high volume semiconductor wafer fabrication techniques. This provides the ability to dramatically reduce the component footprint, avoid multiple packaging issues, eliminate multiple optical alignments and, eventually, achieve the unprecedented cost efficiency and volume scalability in mass production of consumer photonics products.
In the context of applications, the advantages of PIC technology become especially compelling when active waveguide devices, such as lasers and/or photodetectors, are combined with the passive waveguide devices and the elements of the waveguide circuitry, to form a highly functional photonic system on the chip with minimal, preferably just one, optical input and/or output port. Since the active devices, which emit, detect or intentionally alter (e.g. modulate) optical signals by electrical means, usually all are made from artificially grown semiconductors having bandgap structures adjusted to the function and wavelength range of their particular application, such semiconductors are the natural choice for the base material of the PICs. For example, indium phosphide (InP) and related III-V semiconductors are the common material system for the PICs used in optical fiber communications, since they uniquely allow the active and passive devices operating in the spectral ranges of interest, e.g. the 1310 nm, 1490 nm and 1555 nm bands, to be combined onto the same InP substrate.
However, such PIC advantages truly change when we consider optical and electronic integration into a single integrated circuit. In the electrical domain, silicon integrated circuits have been widely adopted in all layers of the network, including physical media drivers, media access controls, and for complex network intelligence functions. In principal, monolithic integration of electronics and optics is possible, can reduce unwanted electrical parasitics, and can allow for a reduction in overall size. Further SiGe alloys allow the provisioning of multi-gigabit digital and multi-gigahertz analog circuits that extend the high speed silicon CMOS into the speed and transmission requirements of these evolving optical networks.
In the optical domain silicon optical circuits with appropriate design, introduction of additional materials such as silicon dioxide, standard electronic dopants, and SiGe alloys selectively enhances the electronic-to-optical interactions, have allowing for the creation of active devices, such as a intensity modulators and photodetectors, as well as passive devices such as optical waveguides and wavelength multiplexers. Further the micro-machining techniques for MEMS allow the inclusion of micro-mechanical elements such as shutters and mirrors into the circuits.
As such it would be particularly advantageous to provide a design approach for wavelength division multiplexers that was compatible with these industry standard processes, materials and techniques such as silicon-on-insulator (SOI) for it's ease of integration to standard silicon CMOS devices and processes. Further it would be advantageous if the design approach supported other materials, such as the previously described III-V semiconductors as well as glasses and polymers.
With monolithic optical and electronic integration the applications of the solutions show promise regarding overcoming technical difficulties in other fields where an extreme amount of data (aggregate bandwidth) is required in a very small space. More traditional applications that would benefit in the future from such optical and electronic integration would be microprocessor data busses, i.e. from microprocessor to memory or between multiple processors in a computer, and in the backplane of multiple microprocessor or server racks. Interestingly these applications would violate a widely held belief that optical communication is the best choice for long-distance transmission (hundreds of meters to hundreds of kilometers) whereas copper traces and copper cables are typically regarded as the best choice in the application space for shorter distances.
Traditionally wavelength multiplexers and demultiplexers were based upon bulk diffraction gratings. Developments in micro-optical variants continue due to the ability to provide devices without temperature control, low polarization dependence and flat passband characteristics. Such developments are detailed in recent publications including Chen et al (U.S. Pat. No. 6,563,977), Cao (U.S. Pat. No. 6,553,160) and Soskind (U.S. Pat. No. 6,735,362). However, they suffer from being discrete passive components that cannot be integrated into a monolithic device for PICs, and involve precise alignment and assembly which becomes very difficult when channel counts increase and tens of optical fibers are aligned to the grating focal plane.
As a result many attempts have been made to reduce the bulk diffraction grating down to a planar form compatible with PICs. For example Cohen et al (U.S. Pat. No. 6,657,723) discloses a planar spectrograph wherein the approach is a hybrid design employing a planar slab waveguide with a diffraction grating etched within which is assembled with micro-optic lens for coupling the multiplexed signal into the slab waveguide and a photodetector array for receiving the demultiplexed wavelength signals.
Extensions of this have integrated a launch/receipt waveguide, allowing direct interconnection with an optical fiber and propagating the multiplexed wavelength stream, and multiple passive waveguides for guiding the discrete wavelength signals into a silica-on-silicon waveguide structure along with the slab waveguide and echelle reflective grating. He et al (U.S. Pat. No. 5,937,113) being a representative example, which additionally includes a polarization compensator. Tolstikhin et al (“Monolithically Integrated Optical Channel Monitor for DWDM Transmission Systems” Journal of Lightwave Technology, vol. 22, no. 1, pp. 146-153, 2004) extended the monolithic integration to include an array of photodetectors by implementing the entire structure within the InP/InGaAsP semiconductor material system.
Common to these approaches is the use of the diffraction effects of a spatial grating, typically utilizing straight or concave spatial grating structures, in a reflection mode. The reflection being achieved by total internal reflection (TIR) at a deep straight wall of high index contrast formed within the integrated demultiplexer structure. The traditional fabrication of such reflective etched grating-based WDM devices generally requires that an etching of a reflective blazed grating is executed through the entire thickness of the waveguide stack structure, i.e. a deep etch of several microns to about 20 microns according to the waveguide material system and structure. This etching creates an air-waveguide interface with a high index contrast barrier for the guided mode in the structure's slab waveguide. The gratings are thus generally reflective through TIR or Fresnel reflections at this barrier. To add additional complexity to the etching, which is very deep by semiconductor processing standards, the grating surfaces must be of high verticality, low surface roughness and be replicated from the mask with sharp corners. All of these constraints further making the etching process complex, expensive and low yield.
Beguin et al (U.S. Pat. No. 6,483,964) discloses the processing issues relating to silica structures wherein the low index requires the additional deposition of coatings, commonly metallic coatings, to provide high reflectivity at the grating facets. With InP or GaAs semiconductor structures the significantly higher refractive index contrast provides possibility of removing the requirement for depositing additional coatings. However, the multiple materials typically present within the InP/InGaAsP and GaAs/AlGaAs structures present additional processing complexities as the etch chemistry employed must work on many materials including in some designs layers normally added to provide etch stops within the structure for easing manufacturing tolerances.
An alternative design approach that has had significant attention is the phased array grating, or array waveguide grating (AWG), such as disclosed by Dragone (U.S. Pat. No. 5,002,350), Dragone (U.S. Pat. No. 5,136,671), and Missey (U.S. Pat. No. 6,728,442). Unlike diffraction grating based structures an AWG is implemented in the waveguide layer as it employs a combination of planar slab waveguides, to provide free propagation zones, and channel waveguides to provide the common multiplexed waveguide and multiple discrete waveguides, together with the large number of waveguides within the central portion of the AWG that provide the phased array. As such the AWG requires etching equivalent to the other optical waveguides in the PIC. For silica this is now only the core layer, typically 5-6 microns, and for InP/InGaAsP material systems etching of a micron or so for rib loaded waveguide structures.
As such AWG structures reduce the manufacturing tolerances and complexity, although providing low polarisation dependence requires careful control of waveguide manufacturing, see for example Parhami et al (U.S. Pat. No. 6,850,670), inclusion of additional elements, see for example He et al (U.S. Pat. No. 5,937,113), as does managing the thermal characteristics of the device, see for example Dawes et al (U.S. Pat. No. 6,519,380).
Additionally researchers have employed directional couplers and ring resonators, as well as experimenting with alternative planar embodiments of alternative bulk optical designs. Amongst the later includes Davies et al (U.S. Pat. No. 5,581,639) wherein a transmissive optical grating is employed within the Raman-Nath regime working on a collimated optical signal launched from an input waveguide in combination with a parabolic mirror. Asghari presenting an alternate transmissive grating absent mirror structures in both reflective (WO 99/60433) and transmissive (WO 99/34539) formats. In Asghari this grating is achieved by etching through the core waveguide layer, whereas Davies et al advantageously specifies that the transmissive grating be formed with shallow etched structures within the upper cladding layer of the waveguide structure.
In order to address the above and other drawbacks of these WDM structures for advanced opto-electronic circuits it would be advantageous to provide a grating structure for a wavelength multiplexer/demultiplexer which is compatible with the material systems of such advanced opto-electronic circuits, such as silicon-on-insulator, standard semiconductor manufacturing processes, such as employed in CMOS, and provides for reduced manufacturing complexity, such as requiring only shallow etching, no metallisation, no verticality requirements on the etching.